WEBVTT

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Voiceover: Did you know
that the stomach flu

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can make you temporarily
lactose intolerant?

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It's true.

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So, in this video, we will
talk about how that happens.

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And in detail, how the
small intestine works.

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So, as a review, once
you put food into your

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oral cavity or your mouth, it's chewed up

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and then sent down the esophagus where

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it ends up in the stomach
where it's churned

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and then introduced to acid where it gets

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broken down into chyme
and then delivered into

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the first part of your small intestine.

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Now, the small intestine has
three different parts to it.

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So, let's take a better look.

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So, the first part of the small intestine

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is called the duodenum, the duo-denum.

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This receives the chyme
that just got processed

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in the stomach and it's
the part of the entire

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GI tract where the most digestion occurs.

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The most breakdown of food products

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will happen in the duodenum.

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All right, so the next part of the

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small intestine is called the jejunum.

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I'll just write that
right here, the jejunum.

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And this is the part
of the entire GI tract

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where the most absorption
occurs, anywhere.

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So, the most absorption of nutrients

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is going to happen in
your jejunum, the jejunum.

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Then, finally after your
food passes to the jejunum,

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it gets to the last part
of the small intestine

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and that's called the ileum,
the ileum (I-L-E-U-M).

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And the ileum, now this
doesn't have a superlative

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like the most digestion
or the most absorption,

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but there are some pretty important

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things that are absorbed here.

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Things like vitamin
B12, vitamin A, D, E, K.

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So, there are some important
things that are absorbed here.

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So, I'm just going to
write important absorption.

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There are some important things

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that are absorbed in your ileum.

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Now, the busiest part
of your small intestine

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is the duodenum, because
there are a bunch of

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things that are involved
in this digestion process.

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So, there are four key
things to keep in mind.

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First of all, your stomach
is going to be delivering a

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bunch of chyme or processed
food into the duodenum.

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So, you're going to be working
with all this chyme here.

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In addition, you're going to have some

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stomach acid that process food into chyme.

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That's going to be
present in the duodenum.

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In addition to the stomach,
the liver and the gallbladder

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are also going to be important

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to deliver bile to your duodenum.

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So, they give bile.

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And as I'll talk about
in a subsequent video,

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bile is composed of two different things.

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Bile salts and bile pigments.

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And beyond the liver and the gallbladder,

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the pancreas also
delivers a couple of very

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important enzymes for digestion here.

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So, I'm just going to
write enzymes for now.

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And in a minute, I'm going
to go through and talk about

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which enzymes are
delivered by the pancreas.

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And then finally the duodenum itself has

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what are called brush border
enzymes, brush border enzymes.

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That are very important for activation of

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certain enzymes and also
for digestion of several

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nutrients that we're going to discuss.

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So, let's talk a little more
about this brush border.

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Now, if I would have make a little

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drawing of the duodenum right here.

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Remember that first part
of our small intestine.

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I would draw just a little
tube, connected right there.

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And then blow up the wall
if I want to take a better

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look at what's going on right there.

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We would find then that there is a whole

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bunch of things than just meets the eye.

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First of all, the wall
isn't just a straight line.

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There is actually a bunch
of infoldings that are

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present on the wall to
help increase surface area.

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Think about it.

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If we're trying to digest
as much as we can here,

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we need to make sure
that there are a lot of

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projections or a lot of
space where we can make

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contact with the food that's passing by.

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So, if this is the inside of our duodenum,

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and this is the outside, just
like how we drew up here.

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That's in and that's out.

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You can notice that this wall here

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has a whole bunch of projections on it.

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These projections are called
villi (V-I-L-L-I), villi.

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And a single one of them is just called

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a villus, just a single villus.

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And these are just a couple
of folds or these outpouchings

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that help increase the
surface area of our duodenum.

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Now, that's not where the story ends.

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If we take a closer look
at one of these villi,

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then we'd find that there
are even more projections

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sitting on that, even smaller
microscopic projections.

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So, for this single villus
that had a horizontal

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line right here for its shape.

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Fair to draw it out here.

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You'd notice that it's
not a straight line,

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but instead these also have a bunch of

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projections that are present on them.

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And if you were to guess why all

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these projections are in there?

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I'm sure you'd say to further
increase our surface area.

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That's sort of the name of the game

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when we're in the small intestine.

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To increase our surface area.

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And so these little guys cutely enough

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are called microvilli, microvilli.

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And a single one is just
called a microvillus.

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So, we've got these villi right here

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or the single villus that you can see

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if you just blow up the
wall of the duodenum.

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And then if you blow up a single villus,

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you'll find that they have a whole bunch

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of microvilli that are found on them too,

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to increase our surface area because

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that allows for better digestion.

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And when we say brush border enzymes,

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these are a whole bunch of enzymes that

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are present on this brush border.

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You guys think about it.

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These villi and these microvilli,

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they're no different
from bristles on a comb.

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They act to increase
surface area or places

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where you're going to have interaction

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with food that you want to digest.

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And so there are enzymes that are

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present on this brush border.

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So, just to make the point, all of these

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microvilli and villi together, that's what

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makes up the brush border of our duodenum.

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The brush border, which is the increased

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surface area of the wall
helps to digest food with our

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brush border enzymes that are present.

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And as we'll see later, in the jejunum,

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it helps for absorption.

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So, now let's talk about the
digestion process in detail.